The failed attempts make it harder to monitor for other attacks because of the noise in log files, network traffic, etc. and if an attacker IP is blocked early they can't try more effective attacks.
[0] https://unix.stackexchange.com/questions/105553/how-to-provi...
edit:
quick and dirty notes of my setup [1]
The SSH daemon logs when it successfully rejects an access. A successfully rejected access is inconsequential to your security. If you are using secure passwords or pubkey authentication, it will never log a successful login by an attacker. What remains then is exploitation of the SSH server ... but the SSH server doesn't have a code path that logs "I have been exploited".